Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of inkjets, which are arranged in one or more printheads, onto an image receiving surface. In an indirect inkjet printer, the printheads eject ink drops onto the surface of an intermediate image receiving member such as a rotating imaging drum or belt. During printing, the printheads and the image receiving surface move relative to one other and the inkjets eject ink drops at appropriate times to form an ink image on the image receiving surface. A controller in the printer generates electrical signals, also known as firing signals, at predetermined times to activate individual inkjets in the printer. The ink ejected from the inkjets can be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, some inkjet printers use phase change inks that are loaded in a solid form and delivered to a melting device. The melting device heats and melts the phase change ink from the solid phase to a liquid that is supplied to a printhead for printing as liquid drops onto the image receiving surface.
During the operational life of these printers, inkjets in one or more printheads may become unable to eject ink in response to a firing signal. The defective condition of the inkjet may be temporary and the inkjet may return to operational status after one or more image printing cycles. In other cases, the inkjet may not be able to eject ink until a purge cycle is performed. A purge cycle can unclog inkjets and return inoperable inkjets to operation. Execution of a purge cycle, however, requires the printer to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of a printer and are typically performed during periods in which the printer is not generating images.
One method to correct image defects produced by an inoperable inkjet includes repositioning the printhead to move another inkjet into a location normally occupied by the inoperable inkjet. The controller operates the other inkjet to substitute for the defective inkjet. The substitution process can completely eliminate image defects due to the inoperable inkjet, but the image receiving member has to rotate past the printhead one or more additional times for the substitute inkjet to print ink drops to correct for the inoperable inkjet. The additional rotations, which also referred to as additional “passes” during printing, reduce the effective throughput of the printer.
Another correction method compensates for an inoperable inkjet by printing additional ink drops from several inkjets that are near the inoperable inkjet in the printhead. The ink drops from the nearby inkjets can camouflage defects that are produced by the inoperable inkjet. The compensating inkjets can operate during normal printing operations so the compensation process does not reduce the throughput of the printer. One drawback of the compensation process is that the ink drops from the neighboring inkjets do not completely correct errors due to the inoperable inkjet. Some printed images can still include noticeable defects even when the printer compensates for the inoperable inkjet.
As described above, existing correction techniques can reduce or eliminate the impact of an inoperable inkjet on printed image quality, but the existing techniques also have drawbacks due to reduced printer throughput or inadequate correction of image defects. Consequently, improvements to the operation of inkjet printers that compensate for inoperable inkjets with a reduced impact to the printer throughput rate, while producing printed images with fewer perceived defects would be beneficial.